Oculoid electronic monitors were devices used for surveillance by the Company. In Megropolis One on Pluto, Gatherer Hade had one track K9 throughout the city. This led him to find the Fourth Doctor, but he was unable to switch targets away from K9 as the system was fixed on the robot.
When the Doctor was captured, Hade placed a track on him before setting him free, thus allowing him to be tracked. However, he was then unable to follow the Doctor into the undercity as no monitors were situated there. Knowing he was being watched, the Doctor had Cordo fetch two monitors and he tampered with them so that they permanently replayed footage of him walking down a corridor.
Hade and his assistant, Marn, noticed this and headed to the location, only to find no one there. They were confused by this and, when asked by the Collector, reported it as a fault. (TV: The Sun Makers)